Analysis methods and systems of such kind are sufficiently known and are used primarily to obtain the most accurate contactless measurement possible of intraocular pressure in an eye. For example, a non-contact tonometer is used for this purpose, with the aid of which a puff of air is applied to the eye being examined, wherein an intensity of the air puff is selected such that the cornea of an eye is pressed inwards, creating a concave surface shape. The cornea briefly forms a flat surface before maximum deformation of the cornea is reached and before the cornea is indented towards the lens of the eye, this surface being called the first applanation point. After maximum deformation of the cornea has been reached and the cornea has returned to its original shape, the cornea passes through a second applanation point of the same kind. Now the intraocular pressure may be calculated by plotting a pressure of the air puff against the development of the corneal applanation over time. The measured values obtained with the non-contact tonometer are set in relation to comparison measured values that have been determined using an applanation tonometer or contact tonometer that produces relatively more accurate measurements, thus enabling a an internal eye pressure to be derived that approximates the actual intraocular pressure more closely as the result.
However, an intraocular pressure that is measured with a non-contact tonometer is not sufficiently accurate compared with a pressure measurement made with an applanation tonometer, because the measurement is distorted by the cornea, among other reasons. In order to improve the measurement accuracy, it was therefore attempted to take the influence of the cornea on the measurement into account, for example with a thickness measurement or measurement of corneal radii before conducting the measurement with a non-contact tonometer. It is also known to consider a modulus of elasticity or Young's modulus as a biomechanical property of the cornea, and to adjust the measurement in question with a corresponding calculation factor. In this context, it is assumed that the modulus of elasticity is always of the same magnitude and is thus constant for all measurements, even for different eyes. It is further assumed that the modulus of elasticity is the same for all areas of a given cornea. Consideration of a modulus of elasticity in a non-contact tonometer measurement has the disadvantage that this material characteristic or material parameter is used to characterise a tensile load, which does not occur with non-contact tonometer measurements. Moreover, a modulus of elasticity varies individually from one eye to the next and also as a function of the respective areas of the cornea within the cornea itself. Therefore, consideration of material parameters of such kind and calculation of a measurement result may still not lead to measurement results of satisfactory accuracy.
It is further known to incorporate the biomechanical properties of a cornea in a non-contact tonometer measurement during the measurement or to calculate these properties as the measurement is being conducted. For this, a puff of air is applied to the cornea, and a pump pressure is recorded continuously during the course of the measurement by a pressure sensor. A timeline of the measurement is also recorded, and first and second corneal applanation points are detected optically. An intraocular pressure may now be derived for example by determining the pressures prevailing respectively at the times of the first and second applanations, particularly since the forces necessary to deflect the cornea both inwardly and outwardly are assumed to be of the same magnitude, and thus cancel one another out. Consequently an intraocular pressure is derived from an average of the force applied for pressing the cornea inwards and outwards, in the form of the air puff.
Alternatively, it is known to determine a hysteresis point between the first and second applanation points and to derive and correct the intraocular pressure on the basis of the hysteresis measurement. In the hysteresis measurement, the first and second applanation points are detected optically and correlated with a timeline of a pressure curve of a pump, that is to say an associated time value and a pressure value is determined for each applanation point. Since the cornea is depressed inwards and the first applanation point is reached at a higher pressure than when cornea is deflected outwards again and the second applanation point is reached, this pressure difference may be used to determine the hysteresis as a material characteristic of the cornea.
The disadvantage of these measurement methods is that a movement of the cornea caused by a puff of air is subject to dynamic effects, which may distort such time/pressure measurements, particularly since the dynamic effects of the described non-contact tonometer measurements cannot be taken into account. In order to avoid such undesirable vibrations of the cornea, a speed of the air puff is minimised as far as possible to avoid distortion of the measurement result due to undesirable movement of the cornea. It is also necessary to synchronise the start of the air puff with the required time measurement. However, when a mechanical pump such as a piston pump is used to generate the air puff, it is not possible to synchronise the times with this degree of accuracy, because of the effects of inertia or friction for example, again leading to a distortion of the measurement result. Moreover, as was indicated earlier, the air puff is pressure-monitored, which means it is altered as required while the measurement is taking place. Thus the air puff is reduced or switched off after the first applanation point has been exceeded to prevent the cornea from being deflected inwards too far. However, this requires continuous monitoring of both the pump pressure and of the course thereof over time relative to the time points of the first and second applanation points, which in turn gives rise to a number of possible sources of error that might distort a measurement result. In summary, therefore, the analysis methods and systems known from the prior art, based on pressure and time measurement systems that operate independently of and parallel with one another with simultaneous detection of the applanation points, are still rather inaccurate compared with a measurement carried out using a contact tonometer.